This invention relates to molten carbonate fuel cells, and, in particular to molten carbonate fuel cell cathodes.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. A fuel cell generally comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. Molten carbonate fuel cells operate by passing a reactant fuel gas through the anode, while passing oxidizing gas through the cathode. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
Molten carbonate fuel cell performance and operating life are dependent in part on the characteristics of the anode and the cathode employed in the fuel cell. For example, fuel cell cathodes need to have excellent conductivity and must have high mechanical strength and durability.
The most commonly used molten carbonate fuel cell cathodes are formed from nickel oxide (NiO) material. Nickel oxide is often preferred to other materials because of its high conductivity at the fuel cell operating conditions. However, the lifetime of nickel oxide cathodes in molten carbonate fuel cells is limited by the dissolution and subsequent precipitation of nickel in the fuel cell electrolyte. More specifically, the presence of carbon dioxide in the cathode gas promotes cathode dissolution, resulting in the following reaction in the cathode:NiO+CO2→Ni2++CO32−
As can be appreciated, the nickel (Ni2+) obtained from the above reaction accumulates and precipitates in the electrolyte matrix. The presence of nickel in the electrolyte matrix creates short circuits, thereby causing a rapid decay in the fuel cell's performance.
Moreover, the properties and performance of molten carbonate fuel cell cathodes are negatively affected by cathode dissolution. In particular, the durability of the cathode decreases considerably as the cathode dissolves. Additionally, NiO dissolution increases the NiO particles of the cathode, therefore significantly reducing the active surface area of the cathode.
Accordingly, it has been recognized that the durability and performance of a molten carbonate fuel cell can be enhanced by reducing the dissolution of its cathode. More particularly, it has been proposed to employ an electrolyte having a more basic pH, such as by increasing the concentration of Li in the Li2CO3/Na2CO3 electrolyte or by adding oxides, e.g. SrO, MgO or La2O3, to the electrolyte, in order to reduce the negative effects of cathode dissolution. In addition, alternative materials for use in fuel cell cathodes, such as LiCoO2, LiFeO2 and Li2MnO3, have been developed which have lower dissolution rates in molten electrolyte.
These methods, however, have not been entirely effective in improving the performance of molten carbonate fuel cells. The addition of oxides such as SrO, MgO or La2O3 to the electrolyte decreases the ionic conductivity of the electrolyte resulting in decreased performance. Also, increasing Li concentration in the electrolyte also increases the melting point of the electrolyte negatively affecting fuel cell performance at low temperatures. Moreover, alternative cathode materials are inadequate for use as molten carbonate fuel cell cathodes due to their low conductivity properties or an insignificant decrease in cathode solubility in the electrolyte matrix at high carbon dioxide pressures.
Mixed oxygen ion conductors, such as ceria and doped ceria compounds, have been used in electrode coatings and fuel cell layers in solid oxide fuel cell applications. For example, U.S. Pat. Nos. 4,702,971 and 4,812,329 disclose a nickel anode for a solid oxide fuel cell having an ionic-electronic conductor coating comprising doped or undoped ceria. In addition, U.S. patent application Publication No. US 2003/0082436 describes an electrode coated with ion conductive ceramic ceria film. More particularly, this application discloses a coated anode for use in a solid oxide fuel cell prepared by dip coating the anode with ceria or doped ceria sol to form an oxygen ion conductive ceramic film.
Moreover, ceria and doped ceria have been used to coat molten carbonate fuel cell anodes. For example, U.S. patent application Publication No. US 2003/0096155 discloses an anode made of an Ni-based alloy or a metal compound for use in a molten carbonate fuel cell coated by a porous ceramic film formed using a sol-gel process. The films described include aluminum oxide sol, cerium oxide sol, cerium hydroxide sol and other compounds.
As can be appreciated from the above, molten carbonate fuel cell cathodes are still needed which exhibit improved properties and, in particular, a higher resistance to NiO dissolution.
It is therefore an object of the present invention to provide a molten carbonate fuel cell cathode which has improved performance characteristics.
It is also an object of the present invention to provide a molten carbonate fuel cell cathode which exhibits reduced dissolution.
It is yet a further object of the present invention to provide a molten carbonate fuel cell cathode having reduced polarization.